1. Field of the Invention
The present invention relates to a sinker electric discharge machining apparatus adapted for generating an electric discharge in a work gap formed between an electrically conductive workpiece and a tool electrode to machine the workpiece. In particular, the present invention relates to a sinker electric discharge machining apparatus that advances the tool electrode attached to a quill towards the workpiece.
2. Description of Related Art
A sinker electric discharge machining apparatus is widely used for manufacturing a mold precisely. Generally, a tool electrode is attached to a quill or a ram that is vertically movable with use of a tool holder. The tool electrode is usually made of copper or graphite. A workpiece is fixed to a surface plate that is disposed in a work tank. During machining, the work tank is filled with a dielectric liquid, and the tool electrode is positioned very close to workpiece. The size of the work gap may range from several micrometers (μm) to hundreds of micrometers (μm).
When a power pulse is applied to the work gap during ON time, insulation characteristics of the dielectric liquid in the work gap is destroyed and electric discharge is generated. A trace amount of material of the workpiece is evaporated or melted due to the heat of the electric discharge. When the ON time ends, the insulation characteristics of the dielectric liquid in the work gap are recovered, and the voltage application is stopped during the OFF time. The electric discharge forms fine crater-shaped holes in a surface of the workpiece. Usually, the sinker electric discharge machining apparatus controls the ON time and the OFF time in a range of 1μ second to several milliseconds and repeatedly supplies a current pulse to the work gap. For most sinker electric discharge machining apparatuses, the tool electrode is lowered along the Z axis towards the workpiece to maintain the work gap a certain size. The trace of material is gradually removed from the workpiece with no contact between the tool electrode and the workpiece. As a result, a cavity that is complementary to the tool electrode in shape is precisely formed in the workpiece.
The process of washing machined debris, removed from the workpiece, away from the work gap is important to the electric discharge machining. A method of creating a flow of the dielectric liquid through the work gap during the machining is known for achieving this purpose. This method is called “flushing.” Flushing prevents undesirable secondary electric discharge that may occur between the machined debris and the tool electrode. Further, the flushing also contributes to the recovery of insulation in the OFF time. A skilled operator may form holes for flushing at appropriate positions on the tool electrode or the workpiece in advance before the machining. Through such holes, clean dielectric liquid can be fed into the work gap and dirty dielectric liquid can be sucked out of the work gap. If the formation of such holes is restricted by the size or shape of the tool electrode, the operator may dispose an injection device at a suitable position for injecting the dielectric liquid to the work gap. The flushing plays a key role in performing electric discharge machining faster and more precisely. However, proficiency is required for creating a uniform and fluent flow in the work gap.
An operation of periodically and rapidly raising and lowering the tool electrode along the Z axis is called “jump.” By performing the jump, almost all of the dirty dielectric liquid in the work gap is expelled from the cavity of the workpiece. If a reciprocating distance of the tool electrode is large, more clean liquid flows into the work gap and more dirty liquid is discharged from the work gap. It is preferable to raise the tool electrode for a distance that is at least equal to or greater than the depth of the hole machined in the workpiece. Because the machining is interrupted during the jump operation, it is preferable to accelerate the jump operation. Due to the use of a linear motor and a quill with reduced weight, a sinker electric discharge machining apparatus capable of performing a high-speed and highly-responsive jump operation has been provided. Such sinker electric discharge machining apparatuses have been disclosed in U.S. Pat. No. 6,353,199 and Japanese Patent No. 4152507.
U.S. Pat. No. 6,353,199 discloses a sinker electric discharge machining apparatus, in which linear motor movers are respectively attached to opposing side surfaces of the quill that is shaped like a quadrangular prism. The quill is made of ceramics. Linear motor stators are attached to a stationary frame. Generally, either of the linear motor movers and the linear motor stators are a row of permanent magnets mounted on a base plate. The base plate is a steel sheet, for example. The other of the linear motor movers and the linear motor stators are yokes with armature coils. The yokes are laminated silicon steel sheets. However, it is not easy to attach the linear motor movers, made of a metal or an alloy, to the nonmetallic quill.
The quill has a hole that extends vertically in the axial center of the quill. A cylinder is disposed in the hole to balance the gravitational load that acts upon the quill which can move in high acceleration. An upper end of the cylinder is fixed to the quill using a flange. A piston is slidably disposed in the cylinder. A stationary piston rod is connected to the piston at one end.
The sinker electric discharge machining apparatus of Japanese Patent No. 4152507 includes a lightweight hollow quill shaped like a quadrangular prism and less affected by the thermal effect. The quill is made of a ceramic sintered body or carbon fiber reinforced plastic. To guide a linear movement of the quill, guide rails are attached to side surfaces of the quill and bearing blocks are attached to the stationary frame. The guide rails and the bearing blocks are generally made of a metal or an alloy.
Compared with the quill, the guide rails thermally expand or contract to a larger extent. Therefore, the quill or the guide rails may bend as the temperature changes. To prevent the bending, it can be considered to form the quill thicker. In addition, it can be considered to attach a dummy member, which has the same shape and the same thermal expansion coefficient as the guide rails, to the inner wall of the quill. However, these prevention measures would lead to an increase of the weight of the quill, which is undesirable.